BELFAST, Northern Ireland — In one of the world’s biggest robberies, thieves took the families of two top bankers hostage and forced the bosses to help them steal more than $39 million from the vaults of a Belfast bank’s main office, authorities said Tuesday.

Experts said Monday’s raid on the Northern Bank cash center was the biggest robbery since 1987, when thieves made off with about $65 million in cash and other valuables from the Knightsbridge Safe Deposit Center in west London.

The tactics in Belfast — particularly the use of hostage-taking as a way to infiltrate a high-security target — suggested a level of sophistication and experience most commonly found within Northern Ireland’s rival outlawed groups, particularly the Irish Republican Army.

“This isn’t a gang of Belfast criminals who just got together. It’s more than that. This looks like a military operation with obvious connotations,” said John O’Connor, a former commander of Scotland Yard’s elite detective unit in London.

Assistant Chief Constable Sam Kinkaid, the police officer leading the investigation, said his detectives didn’t yet know whether a particular group was responsible for what he called a “clearly well organized” raid.

In the neighboring Irish Republic, Justice Minister Michael McDowell said peacemaking efforts could be hurt if police linked the raid to the IRA, which is known to have robbed banks in the past to finance operations.

The IRA has observed a truce since 1997 but remains active, running criminal rackets such as smuggling fuel and cigarettes. Diplomatic efforts to revive a power-sharing government in Northern Ireland involving Sinn Fein, the IRA-linked party backed by most Roman Catholics, have repeatedly stumbled over other groups’ demands that the IRA disarm and disband.

Police didn’t learn about the heist until three hours after the robbers had left in a truck filled with cash from underground vaults at the bank’s downtown headquarters.

Kinkaid said masked gunmen invaded the homes of two senior employees of Northern Bank late Sunday and warned that if the executives didn’t cooperate or tried to raise an alarm, their families would be killed.

The families were held at gunpoint at undisclosed locations outdoors where overnight temperatures were near freezing.

About 6 p.m. Monday, Kinkaid said, the robbers began clearing out vaults packed with cash ready to be distributed to the bank’s 95 branches and hundreds of automated teller machines across Northern Ireland.

O’Connor said the gang researched Northern Bank’s security systems expertly. He said the two bank officials probably were targeted because there were “two lots of combinations on the vault, so no one person has the knowledge.”

He said the gang must have had more than a dozen members, with at least four guarding each hostage family. Such a gang “must be confident with each other that they’re all staunch and they won’t roll over in the event of being nicked (arrested),” he said.

Both families “suffered great trauma” during their abduction but were released unharmed, except for one person who needed treatment for hypothermia, he said.

Kinkaid said the bank hadn’t been able to provide an exact figure for the amount of stolen, partly because police shut down its offices for forensic examination, but he said it might exceed 20 million pounds, or about $39 million.

Other security officials said the vaults held closer to $58 million because of the heavy cash needs of the Christmas holidays.

The Guinness Book of World Records lists the theft of gold bullion from the central bank of Nazi Germany in 1944 and 1945 as the biggest of all time, valued in 1984 at $4 billion. The largest “normal” bank robbery is listed as the 1976 theft of $50 million in cash and deposit-box valuables from a Lebanese bank.

The IRA previously gained access to high-security targets by taking the families of employees hostage, most infamously in October 1990, when it forced civilian employees of the police to drive car bombs to three British military installations. Six soldiers and a chef died in the remote-control blasts.

Police say the IRA still uses hostage-taking in its criminal operations. They cite an incident in May when the IRA was accused of taking staff hostage at a retail superstore, then stealing more than $7.75 million in alcohol, appliances and other goods.